The present invention relates to a distance measuring device using two kinds of frequencies, and more particularly to an electro-optical distance meter for measuring a distance to a reflecting mirror located at a measuring point by combining the results of fine measurement and rough measurement wherein the fine measurement is carried out by detecting a phase difference between a modulation signal generated from light emitting means and a reflected modulation signal reflected by the reflecting mirror and received by light receiving means, while the rough measurement is carried out by measuring a lag time between the modulation signal and the reflected modulation signal.
The present invention further relates to an electro-optical distance meter for measuring a distance by detecting a phase difference and a lag time of a multiplex modulation signal, and more particularly to a distance measuring device which can carry out the measurements of the phase difference and the lag time in parallel with use of a single frequency.
A conventional electro-optical distance meter is constructed of a body of the distance meter and a reflector. A modulation wave generated from the body is reflected by the reflector, and a reflected modulation wave from the reflector is received by the body. A phase difference between the modulation wave from the body and the reflected modulation wave from the reflector corresponds to a distance between the body and the reflector. Therefore, the distance between the body and the reflector can be calculated by detecting the phase difference.
In case of measuring a short distance, the reflector such as a reflecting mirror may not be specially provided, but an object located at a measuring point may be utilized as the reflector.
In general, the electro-optical distance meter requires a high accuracy of measurement, and it is therefore necessary to use a modulation wave having a relatively short wavelength for the measurement. For instance, in case of obtaining a measurement accuracy of about .+-.5 mm, it is necessary to use a modulation wave having a first wavelength .lambda..sub.1 =20 m for fine measurement. However, the relation between a distance to be measured and a phase of the modulation wave is such that the phase changes in the range of 0-2.pi. with a change in the distance of every 10 m (optical path length of 20 m) Accordingly, the distance of 10 m or less can only be measured (i.e., a measuring period is 10 m). To measure a distance more than 10 m, it is necessary to use a modulation wave having a second wavelength .lambda..sub.2 larger than the first wavelength .lambda..sub.1 for the fine measurement and thereby carry out rough measurement (approximate measurement). Further, to measure a longer distance, it is necessary to use a modulation wave having a third wavelength .lambda..sub.3 larger than the second wavelength .lambda..sub.2 for the rough measurement and thereby carry out very rough measurement.
The results of the fine measurement, the rough measurement and the very rough measurement are combined to obtain a final measurement result. That is, the distance measurement is carried out with use of three kinds of frequencies corresponding to the three kinds of wavelengths .lambda..sub.1, .lambda..sub.2 and .lambda..sub.3.
However, as the conventional electro-optical distance meter uses three or more kinds of frequencies for the measurement, it is necessary to time-divisionally select the wavelengths, causing an increase in total measuring time. To shorten the total measuring time, it is necessary to shorten a partial measuring time corresponding to each measuring frequency, causing a reduction in the measurement accuracy.
Furthermore, the partial measuring times corresponding to the respective measuring frequencies are different from one another to result in a difference in turbulent influence by the atmosphere or the like. This difference causes an error in calculation of figure arrangement of each measuring wavelength.
In these circumstances, it has been greatly demanded to realize an electro-optical distance meter which can shorten the total measuring time without reducing the measurement accuracy.